There is a growing interest in making light sources colour-tunable. Normally, it is proposed to combine LEDs with different colour spectra in one lighting product.
Some lighting products generate different spectra at different positions, and sometimes also in different directions. Depending on the optical architecture, this may result in visible colour differences in the light distribution in space (in the far field distribution), in visible colour differences in the emitting area in the luminance distribution (which is also called the near-field distribution) and in coloured shadows.
Other disadvantages relate to sizes of optical elements used in such lighting products. Current colour-mixing optical elements are either very large to achieve good mixing, or constrained in size (to reduce cost, or to keep a sufficiently high brightness) with a compromise between near-field colour mixing quality and far-field colour mixing quality. Moreover, for small mixing chambers, substantial efficiency losses may occur.
The known solutions so far regarding light sources having different spectral emission are to position the light sources very close together to minimize the size of the colour-mixing optical element. However, this is also disadvantageous for thermal management reasons.
Thus, there is a need for a colour-tunable, less complex, low-cost lighting device that has an improved colour mixing quality.
U.S. Pat. No. 8,953,926 discloses a light emitting device based on a light guide structure. The light emitted from the light sources (LEDs) is injected into the light guide and transported via internal reflection to the places where it is emitted to the ambient.
US 2015/036333 discloses a light emitting device in which at least part of the light is injected in a light guide that is also arranged as envelope for emitting the light to the ambient; the remaining part is transmitted into the interior volume.
US 2013/039058 shows a lighting device in which the light is injected into a light guide and distributed in this light guide by internal reflections and then emitted to the ambient.